I. Field of the Invention
The present invention relates to devices for machining parts, and more particularly to an apparatus for machining main bearings, crankpins, flanges, cheeks and the like of the crankshaft of an internal combustion engine, or machining any part having a round outside diameter on face.
II. Description of the Prior Art
The crankshaft is a well known element of a reciprocating piston engine or compressor. The crankshaft includes a plurality of main bearings on whose centers the crankshaft rotates, and a plurality of crankpins offset from the axis of the crankshaft. The pistons of the engine are connected to the crankpins by a plurality of connecting rods. The crankshaft thus changes the linear motion of the pistons to rotational motion, which is then transmitted to a drive means, transmission or the like.
Due to the high rotational speed and stresses encountered during operation, the crankshaft is forged metal (typically steel or ductile iron) for strength, and it is critical that the crankshaft be machined with great precision. The elements to be machined include the counterweight faces (cheeks) and diameters; the main bearing and crankpin bearing faces (sidewalls and thrust walls), diameters and undercuts; and other faces and diameters such as gear fits, oil seals, pulley fits, slingers, flanges, gears and reluctors.
Several methods are known for machining crankshafts from rough forgings or castings. However, each of the known methods has a number of associated drawbacks. Machining of the crankshafts by turning allows little flexibility for tooling changeover, requires frequent tool change and other maintenance, has poor swarf control and entails a high sound level in the shop. In contrast, machining by skiving (and/or turn broaching) has a high initial and rebuilding cost, requires a good deal of floor space, has no steady rest capability, and has little flexibility in machine configuration. Milling has also been employed for machining crankshafts, but does not readily allow the combining of several operations on the workpiece at the same location, and at the same time entails relatively expensive maintenance, is not useful for a number of crankshaft operations, requires special premachining of the crankshaft, and also has a relatively high initial and rebuilding cost. A last known method, grinding, is not often used to machine crankshafts because of high initial and rebuilding costs, low production capability, a high cost in perishable tools, significant tool change downtime, an inability to combine several operations, and an inability to achieve optimal cutting speeds, feed rates and tool dimensional tolerances. The relatively high cost of machining crankshafts is a significant factor in the cost of manufacturing an engine, and therefore in manufacturing a motor vehicle. Indeed, one major cost and inefficiency in prior methods of manufacture has been the need to withdraw or reciprocate the machining edge, tool or implement from the area of the workpiece, before positioning and working a new workpiece.